- BIT DEPTH TUTORIAL -

Bit depth quantifies how many unique colors are available in an image's color
palette in terms of the number of 0's and 1's, or "bits," which are used to
specify each color. This does not mean that the image necessarily uses
all of these colors, but that it can instead specify colors with that level
of precision. For a grayscale image, the bit depth quantifies how many
unique shades are available. Images with higher bit depths can encode
more shades or colors since there are more combinations of 0's and 1's available.

TERMINOLOGY

Every color pixel in a digital image is created through some combination
of the three primary colors: red, green, and blue. Each primary color
is often referred to as a "color channel" and can have any range of intensity
values specified by its bit depth. The bit depth for each primary color
is termed the "bits per channel." The "bits per pixel" (bpp) refers to
the sum of the bits in all three color channels and represents the total colors
available at each pixel. Confusion arises frequently with color images
because it may be unclear whether a posted number refers to the bits per pixel
or bits per channel. Using "bpp" as a suffix helps distinguish these two
terms.

EXAMPLE

Most color images from digital cameras have 8-bits per channel and so they
can use a total of eight 0's and 1's. This allows for 28 or
256 different combinations—translating into 256 different intensity values for
each primary color. When all three primary colors are combined at each
pixel, this allows for as many as 28*3 or 16,777,216 different colors,
or "true color." This is referred to as 24 bits per pixel since each pixel
is composed of three 8-bit color channels. The number of colors available
for any X-bit image is just 2X if X refers to the bits per pixel
and 23X if X refers to the bits per channel.

COMPARISON

The following table illustrates different image types in terms of bits (bit
depth), total colors available, and common names.

Bits Per Pixel

Number of Colors Available

Common Name(s)

1

2

Monochrome

2

4

CGA

4

16

EGA

8

256

VGA

16

65536

XGA, High Color

24

16777216

SVGA, True Color

32

16777216 + Transparency

48

281 Trillion

BIT DEPTH VISUALIZATION

By moving your mouse over any of the labels below, the image will be re-displayed
using the chosen amount of colors. The difference between 24 bpp and 16
bpp is subtle, but will be clearly visible if you have your display set to true
color or higher (24 or 32 bpp).

24 bpp

16 bpp

8 bpp

USEFUL TIPS

The human eye can only discern about 10 million different colors, so
saving an image in any more than 24 bpp is excessive if the only intended
purpose is for viewing. On the other hand, images with more than 24
bpp are still quite useful since they hold up better under post-processing
(see "Posterization
Tutorial").

Color gradations in images with less than 8-bits per color channel can
be clearly seen in the
image histogram.

The available bit depth settings depend on the file type. Standard
JPEG and TIFF files can only use 8-bits and 16-bits per channel, respectively.